The student's main argument is essentially the following:
Libertarians are against redistribution. Redistribution takes money from the rich and gives it to the poor. When you don't redistribute, the rich will get more and more wealth and the poor, comparatively, will get less and less.
I don't see anything wrong with the OP responding to this argument. As far as the letter writer is concerned, he is seeking truth, and he was willing to write a letter on the subject. This seems like a perfect opportunity for discussion.
People aren't going to come up to us and say, "oh wise one, help me understand the truth as you understand it". They are going to come with their own preconceived ideas, biases, etc. Just like we do when the shoe is on the other foot.
The Daily Paul